The present invention is directed to a powder injector for forming a mixture of powder and air and feeding the mixture to a powder coating gun. Particularly, the present invention relates to a powder injector for forming a powder-air mixture wherein the powder injector comprises a powder suction conduit, an injector inserted in said powder suction conduit, a venturi tube connected thereto, the venturi tube being provided downstream of the injector, a conveying air supply conduit having a control valve therein for feeding conveying air to the injector, and a control air supply conduit for feeding control air to control air ports opening into the venturi tube. Powder injectors of this type are known and are being used in powder coating plants. Such powder injectors are disclosed in German Patent DE 37 21 875 A1 and corresponding U.S. Pat. No. 4,941,778 herein incorporated by reference.
In the known powder injector of the specified kind, conveying air and control air or "metering" air are set or adjusted independently of one another by the user by means of control valves associated with the conveying air supply conduit and the control air supply conduit. Adjustment of the conveying air will cause a variation of the powder quantity which is drawn in and conveyed to the coating gun and also of the flow rate of the powder-air mixture in the supply conduit to the powder coating gun. In addition to the conveying air, the metering air affects the powder concentration in the powder-air mixture and, above all, the velocity of the powder-air mixture in the supply conduit leading to the coating gun.
It is generally known that for a small quantity of conveying air and hence a low conveying air pressure, i.e., when small powder quantities are required, increased amounts of metering air and hence a higher metering air pressure are required because otherwise the velocity of the powder-air mixture in the supply conduit to the coating gun will be too low. An inadequate velocity results in the precipitation of powder from the mixture, non-uniform concentration, and the occurrence of pulsations. On the other hand, for medium pressures of conveying air, comparatively little metering air is required, and for higher pressures of conveying air a constant basic quantity of metering air will suffice.
As described above, the conveying air and metering air are set or adjusted independently of each other by the user, that is, the user initially sets the conveying air in such a way as to cause conveying of the desired powder quantity whereupon the metering air quantity is set "intuitively" in a way deemed proper by the user in view of the predetermined conveying air quantity. An optimum relationship between the metering air quantity and the conveying air quantity can only rarely be achieved in this way, and there may also occur fluctuations in the conveying air pressure so that in operation the user will frequently have to readjust the metering air even though there is no variation in conveying air. Finally, such mutually independent setting and adjusting of conveying air and metering air is tedious and requires the manipulation of an experienced operator.